Unfortunately, there is no quick & easy solution to the problem of connecting the main board to test it under power. In my Siesta article at
http://antiqueradio.org/PhilcoH3412L...Television.htm you can read about my investigations into permanently installing quick-disconnect connectors. In the end, I decided they would create too much clutter. Using a bunch of clip connectors for the temporary hookup might work, if you don't mind investing the time & money for a bunch of connectors you may never use again.
An alternative is to do a very thorough restoration of the board while it is out, checking every single resistor, and so on. The disadvantage is that you are replacing a large number of parts with no opportunity to test the TV along the way. Sometimes the symptom caused by an inadvertent miswiring can be very puzzling to diagnose. Classical diagnostic methods (understandably) take the approach that you're trying to locate a failed part in a TV that is correctly wired. If you accidentally wire the neck bone to the ankle bone, so to speak, the symptoms may be rather bizarre.
Be aware that the board has multiple, separate ground connections to the main chassis. There is no single "ground" shared by the entire board. These separate grounds are made through lugs on the main chassis, and they are sometimes troublesome. On one of my Predictas, I added a ground line connecting all of the grounds on the board, to minimize this problem.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html